Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity and are generally poor at incorporating α-olefin comonomers. To improve polymer properties, single-site catalysts, in particular metallocenes are beginning to replace Ziegler-Natta catalysts.
Catalyst precursors that incorporate a transition metal and an indenoindolyl ligand are known. U.S. Pat. Nos. 6,232,260 and 6,451,724 disclose the use of transition metal catalysts based upon indenoindolyl ligands.
U.S. Pat. No. 6,559,251 discloses a process for polymerizing olefins with a silica-supported, indenoindolyl Group 4-6 transition metal complex having “open architecture.” WO 01/53360 discloses similar open architecture indenoindolyl catalysts that may be supported on an inert support. U.S. Pat. No. 6,211,311 teaches that many heterometallocenes are inherently unstable and this causes difficulties in supporting these catalysts and poor catalyst activity. This problem is avoided by using chemically treated supports to prepare supported catalysts containing heteroatomic ligands.
U.S. Pat. No. 6,908,972 discloses a process for polymerizing ethylene in the presence of a silica supported Group 3-10 transition metal catalyst that has two bridged indenoindolyl ligands. The catalyst is effective for copolymerizing ethylene with α-olefins such as 1-butene or 1-hexene to make low density polyolefins.
Mixed catalysts are known. For example, U.S. Pat. No. 4,701,432 codeposits two catalysts—a metallocene catalyst and a non-metallocene Ziegler-Natta transition metal compound—on a support and uses the mixed catalyst system to polymerize ethylene. Indenoindolyl transition metal complexes are not used. While titanium alkoxides fall within the broad disclosure of possible non-metallocene compounds, each of the examples uses a halogenated titanium compound such as titanium tetrachloride or di-(n-butoxy)titanium dichloride. Similarly, the reference discloses that organic compounds of lithium, calcium, zinc, and aluminum can be combined with the catalyst component, and many alkyl aluminum compounds are listed including some branched alkyl aluminum compounds. However, no preference is given for branched alkyl aluminum compounds. The examples all use a combination of methylalumoxane and trimethylaluminum. Nothing indicates that the mixed catalysts give polyolefins having decreased density. The examples in which ethylene alone is polymerized provide polyethylene with a density of 0.960 g/cm3. Lower densities are only obtained when 1-butene is used as a comonomer.
Mixed catalysts are used to give low density polyethylene in J. Appl. Polym. Sci. 94 (2004) 2451. Indenoindolyl transition metal complexes are not used. No branched alkyl aluminum compounds are used. Only a combination of methylalumoxane and triethylaluminum is used.
Despite the considerable work done with catalysts based upon indenoindolyl ligands, there is a need for improvement, especially with regard to making low density polyolefins. Copolymerization of ethylene with an α-olefin lowers density but adds to the cost because the α-olefin is normally more expensive and requires separate equipment. It would be desirable to make polyethylenes having low density from ethylene alone.